Multi-mode Lamb wave tomography with arrival time sorting.

Lamb wave tomography has been shown to be an effective nondestructive evaluation technique for platelike structures. A series of pitch-catch measurements between ultrasonic transducers can be taken from different orientations across the sample to create a map of a particular feature of interest such as plate thickness. Most previous work has relied solely on the first arriving mode for the time-of-flight measurements and tomographic reconstructions. The work described here demonstrates the capability of the Lamb wave tomography system to generate accurate reconstructions from multiple modes. Because each mode has different through-thickness displacement values, each is sensitive to different types of flaws, and the information gained from a multi-mode analysis can improve understanding of the structural integrity of the inspected material. However, one of the problems with the extraction of multi-mode arrival times is that destructive interference between two modes may cause one of the modes to seemingly disappear in the signal. The goal of the sorting algorithm presented in this work is to try and counteract this problem by using multiple frequency scans--also known as frequency walking--to sort the arrival times into their correct mode series.